To further our understanding of the molecular mechanism of mammalian fertilization, we are studying the structure of murine sperm chromatin and the molecular processes of nuclear decondensation that occur after sperm penetration of the egg. Using endonucleases as probes, we have documented the resistance of the condensed sperm DNA to nucleolytic digestion. This resistance is markedly diminished after the induction of in vitro decondensation with reducing agents and proteolytic enzymes. Using our ability to decondense sperm chromatin in vitro we are assaying for oocyte macromolecules that mediate the decondensation reaction.